drm/i915: make enable/disable_gt_powersave locking consistent

The enable functions grabbed dev->struct_mutex themselves, whereas
the disable functions expected dev->struct_mutex to be held by the
caller. Move the locking out to the (currently only) callsite of
intel_enable_gt_powersave to make this more consistent.

Originally this was prep work for future patches, but I've chased down
a totally wrong alley. Still, I think this is a sensible
clarification.

Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>